1. Technical Field of the Invention
The present invention relates to satellite telephone communication systems, and more particularly, to a method for acquiring a spotbeam beacon frequency within a satellite communications system.
2. Description of Related Art
The next area of major development within wireless communication systems will likely involve the use of satellite telephones. Satellite-cellular communication systems, such as the Association of SouthEast Asian Nations' (ASEAN) Cellular Satellite (ACeS) system which is designed to provide telephone coverage by use of a geostationary satellite, have been proposed which, when implemented, shall permit a user to communicate telephonically by way of the satellite-cellular communication system when positioned at almost any location. By transmitting down-link signals between a satellite-based transceiver and the radiotelephone and up-link signals between the radiotelephone and the satellite-based transceiver, telephonic communication shall be possible between the radiotelephone and the satellite-based transceiver. By effectuating additional communication links between the satellite-based transceiver and a ground station, the user of the radiotelephone shall be able to communicate telephonically with another party by way of the ground station and the satellite-based transceiver.
Because of the inherent efficiencies of digital communication techniques, many already-installed cellular communication networks have been converted, and many newly-proposed cellular communication systems, such as the ACeS system, are being designed, to utilize digital communication techniques. Other communication systems similarly utilize, or are planned to be converted to or to utilize, digital communication techniques.
To function properly, particularly when the communication system utilizes digital communication techniques, the radiotelephone must be synchronized with a network station of the cellular communication network. Conventionally, synchronization signals are transmitted by the network station to the radiotelephone to synchronize the radiotelephone with the network station. Other communication systems similarly utilize conventional, synchronization signals for similar purposes.
In a TDM communication system, such as a time division multiple access (TDMA) system, communication is effectuated by the use of frames. In TDMA, a given frequency band is divided into a series of discrete frames each having a series of discrete timeslots therein, each timeslot for use by a different subscriber. Although many systems utilize eight timeslots per frame, ACeS provides for multiple users per time slot, effectively becoming a 16- or 32-slot system. During each timeslot, information may be transmitted in burst form in accordance with a particular multiframe configuration. A normal burst is the transfer of speech or data information. Other burst types include high-power synchronization bursts, groups of which form the aforementioned synchronization signals, which are preferably unevenly spaced across many frames within a multiframe, i.e., 102 consecutive frames in the ACeS system. The initial high-power synchronization burst in such a signal, however, is placed at the start of the first frame in a multiframe, signaling the multiframe boundry, and the remaining synchronization bursts, usually three, which could constitute high-power broadcast bursts, are unevenly spaced from the initial sync burst within the multiframe by known offsets.
It is readily apparent, however, that a radiotelephone or cellular phone, upon initial power up, is not synchronized with the digital bit stream emanating from the transmitter and must ascertain the multiframe boundry, i.e., the start of the initial synchronization burst, within that bit stream. Once the first sync burst and the multiframe boundry are found, the receiver may then quickly get in sync with the transmission. However, many multiframes of the transmission may transpire and a significant number of processing steps performed before synchronization is achieved even without the presence of interfering signals.
Conventionally, two types of synchronization are performed: coarse and fine. Coarse synchronization is designed to narrow the bit stream selection to a particular portion of consecutive bits hopefully containing the initial high-power sync burst, i.e., the multiframe boundry. Fine synchronization then determines the exact location of the initial sync burst within that portion by correlating or matching a segment of the selected consecutive bits to a bit pattern, and shifting the segment bit-by-bit until correlation and synchronization are achieved.
Control channels between an orbiting satellite and a mobile station telephone are enabled through a beacon channel providing general information to a mobile station and enabling initial carrier acquisition. Interaction between the satellite and the mobile station requires that the mobile station acquire a fix upon the beacon frequency associated with the spotbeam within which the mobile station is located. The spotbeam comprises a focused antenna pattern transmitted from a satellite to a limited geographical area. The spotbeam enables transmission of signals from a satellite to a well-defined area in which the mobile station is located.
Acquisition of the spotbeam by a mobile station enables initiation of connection procedures. Connections occur via a short or normal spotbeam selection process. For a short spotbeam selection process which utilizes prestored information or user interaction within an ACeS system, acquisition times of up to two minutes are allowed. For a normal spotbeam selection process which does not utilize any prestored spotbeam information or user interaction, acquisition times of up to twelve minutes are allowed.
Due to a lack of coordination of frequency planning throughout satellite communication service areas, up to 140 different frequencies for an ACeS system (the number of frequencies may vary depending on the system used) may be used as a spotbeam beacon frequency. Each spotbeam may have a unique beacon frequency. Thus, for normal spotbeam beacon frequency selection processes, the mobile station must search through over 140 different beam frequencies to determine the beacon frequency for the mobile station.
Evaluation of existing initial coarse synchronization procedures within an ACeS system reveal that in order to provide a 28 dB link margin over an additive white guassian noise (AWGN) channel, five multiframes of sync time are required to achieve a better than 99% coarse synchronization success rate. Proposed synchronization schemes require 114 seconds for short spotbeam selection processes and 729 seconds for normal spotbeam selection processes. Though the sync time achieved by existing schemes meets the short spotbeam selection process requirements and is close to meeting normal spotbeam selection process requirements, there is a need to significantly reduce the sync times to make satellite mobile stations even more effective.